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Fixed Fire Fighting Systems in Road Tunnels
An overview of current research, standards, attitudes, and debates

By Andreas Haggkvist | Fire Protection Engineering

Using any
type of Fixed Fire Fighting Systems (FFFS) in road tunnels has, in many
parts of the world, been a controversial issue and still is today. An
overview is presented regarding the current research, standards,
debates, and attitudes regarding FFFS in road tunnels. This overview
shows that there is a general negative viewpoint at using FFFS from the
decision makers, which to a large part contradicts from the findings of
research, results and experience from actual tunnel fires.

INTRODUCTION

The
purpose of a FFFS in a road tunnel is determined by the fire protection
objectives, which can be simplified to either fire suppression or fire
control. The differences between the two objectives are the amount of
water needed to be used, the possible use of a foam agent, the
positioning of the sprinklers and the principle of the activation
system.1

The fixed fire
fighting systems used in road tunnels are generally water based. The
different kinds of water-based FFFS can be divided into three groups:
traditional sprinklers/water spray systems, water mist systems and
systems with an added foam agent. In principle, it can be said that
smaller droplets will attack the flames and the smoke plume while larger
droplets will attack the fire base and solid objects. However, adding a
foam agent to the FFFS will create a film of foam which will separate
the burning fuel from the oxygen and thereby suffocate the fire.

USEFUL STANDARDS

Two standards that can be used when engineering FFFS inroad tunnels are NFPA 5022 and Engineering Guidance for Water-Based Fire Fighting Systems for the Protection of Tunnels and Subsurface Facilities.3 As the title implies, the NFPA 502 is not exclusively intended as a standard for road tunnels and FFFS.

During the European Research Project UPTUN, the Engineering Guidance for Water-Based Fire Fighting Systems for the Protection of Tunnels and Subsurface Facilities was
written. It provides information on design, installation and
maintenance of water-based Fixed Fire Fighting Systems to be used in
tunnels.

ROAD TUNNELS WITH FFFS

Japan
has suffered from accidents involving tunnel fires, and this has
resulted in a unique experience using FFFS for a period that has
extended over more than four decades.4 They have approximately 80 road tunnels equipped with FFFS.5

Some road tunnels in the rest of the world equipped with FFFS are shown in Tables 1 to Table 3.

LARGE-SCALE FIRE TESTS

The main objective of the UPTUN project5 was
to find new methods for fire safety in existing tunnels, and during
this project, tests with water mist systems were performed. The
large-scale fire tests were focused on fire control rather than fire
suppression, and they were conducted with low pressure and high pressure
water mist systems. Two different fire scenarios were used in the
tests: pool fires and solid fuel fires in the form of a stack of wood
pallets. The fires had a potential severity on the order of 10 MW to 20
MW under free burning conditions. Some results from the UPTUN project
were as follows:

Both types of systems tested were able
to reduce the heat release rates of the fires in the range of 40% to
70%. It was not possible to determine whether or not one type of system
performed better than the other.

After the activation of the systems, gas temperatures were reduced rapidly downstream the fire.

Back layering was reduced after the
activation of the system, which resulted in better visibility upstream
the fire.

The efficiency of the system was dependent on the fire size,
nozzle type, water discharge density and the location of the fire.

The visibility downstream the fire did
not initially improve; however, when the fire size was reduced by the
water mist system, the visibility also increased.

In
2005, a series of tests was conducted in the Runahamar Tunnel in Norway
in order to evaluate the effectiveness of FFFS using compressed air
foam (CAF).6 The first fire test consisted of solid fuel in the form of wood pallets with a volume of 100 m3 and with a heat release rate (HRR) up to 300 MW. The second test fire was a diesel pool fire with an area of 100 m2 and
a HRR of 200 MW. The FFFS successfully extinguished the large pool fire
and controlled the solid fuel fire, but did not extinguish it. Upstream
of the fire, the air temperature was cooled down to 50˚C and downstream
the temperature was cooled to below 100˚C. Thus, preventing fire spread
and generating an acceptable working environment for firefighters. The
visibility in the tunnel was completely lost before the discharge of the
FFFS.

RESERVATIONS AGAINST THE USAGE OF FFFS IN ROAD TUNNELS

There
are numerous reservations against the usage of FFFS in road tunnels.
These reservations have been made in the past and are still used today.

One
concern has been that using FFFS in road tunnels could worsen the
conditions for the evacuating tunnel users. This is because the FFFS
will generate steam that may injure them or that the cooling effect of
the FFFS could cause destratification of the smoke.

Other concerns have
focused on the effectiveness of FFFS in road tunnels. Fires that start
in the engine or in the compartment may be difficult to protect using a
FFFS. Another concern has been that petroleum fires may continue to
produce combustible gases after they have been extinguished by a FFFS,
and an explosive environment will be created.

Current
research shows that FFFS lowers temperatures in the tunnel
considerably, even close to the fire. A free burning fire can reach
1000˚C at relatively long distances from the fire source. Steam can, in
some cases,be found in the close proximity of the fire but the cooling
effect of a FFFS outweighs any danger that might be caused by this.3

Concerning
the potential destratification of the smoke, research shows that the
smoke will reach the tunnel floor when using these kinds of systems.
However, tests conducted within the UPTUN project have shown that the
smoke remains stratified only for relative short distances even when
FFFS are not used due to thermal effects and ventilation.

The
FFFS also reduce the production of smoke, and the water droplet itself
can bind particles -thereby reducing the toxic effects and increasing
visibility conditions.3

Concerning
the effectiveness of FFFS, the general view today on FFFS is that they
are used to suppressor control the fire, stopping it from spreading, and
to reduce temperatures in the tunnel. The main reason for installing a
FFFS is, therefore, not to extinguish a fire. The intention is that the
fire is later extinguished by the rescue services.

If
a fire in a vehicle is not suppressed or controlled, it will in a very
short time over tax the vehicle and start spreading itself to adjacent
vehicles. Furthermore, it is more vital to protect the evacuees from the
high temperatures and the high HRRs generated than a potential
explosive environment after a petroleum fire has been extinguished by
the rescue services. In most cases, people who can escape have done so.

About SFPE

SFPE is a global organization representing those practicing in the fields of fire protection engineering and fire safety engineering. SFPE’s mission is to define, develop, and advance the use of engineering best practices; expand the scientific and technical knowledge base; and educate the global fire safety community, in order to reduce fire risk. SFPE members include fire protection engineers, fire safety engineers, fire engineers, and allied professionals, all of whom are working towards the common goal of engineering a fire safe world.